A related art reference of the present invention will be described with reference to an active matrix type liquid crystal display device having a display pixel electrode array that is constructed of thin film transistors (thereinafter referred to as TFTs) as switching devices.
The active matrix type liquid crystal display device comprises an array substrate, an opposite substrate, and a liquid crystal material. A display pixel electrode array is formed on the array substrate. An opposite electrode is formed on the opposite substrate. The liquid crystal material is disposed between the array substrate and the opposite substrate. The TFTs and display pixel electrodes connected thereto are formed on the array substrate in a matrix shape. In addition, scanning lines are connected in common to gates of the TFTs in the column direction of the matrix. Signals lines are connected in common to drain electrodes of the TFTs in the row direction of the matrix. Moreover, capacitive lines and so forth are disposed opposite to the display pixel electrodes through an insulation layer.
These electrodes and semiconductor devices such as TFTs are conventionally fabricated by thin film pattern forming technologies, namely photofabrication technologies.
In a conventional thin film pattern forming process, a thin film material is formed on a substrate by a particular film forming method such as spattering method or CVD method. The thin film is patterned in a desired shape by so-called photoetching process (PEP).
In other words, a photoresist is coated on the thin film formed on the substrate. The photoresist is developed in a predetermined pattern by an exposing process. In other words, a photomask having a light insulating member with a predetermined pattern is aligned on the upper surface of the substrate. Rays of light are exposed to the substrate through the photomask.
Thereafter, the exposed photoresist is developed. With a mask of the developed photoresist, the undesired portion of the thin film formed on the substrate is etched out and a desired pattern is obtained. By repeating these processes the number of times corresponding to the number of layers of thin films that construct the electrodes and semiconductor devices, a desired device can be fabricated.
As the areas of optical devices such as liquid crystal display devices increase, needs of thin film forming technologies and patterning technologies for fabricating their display devices are becoming strong.
For example, when the exposing process is performed, since an optical portion of the exposing device has a restricted performance, the area of the substrate that is exposed by the exposing device at a time is restricted. To expose a large area of a substrate, so-called division exposing (stepper) method is employed.
In the division exposing method using the stepper, the area of the substrate to be exposed is divided into a plurality of exposure areas as shown in FIG. 11. The exposing device performs the exposing process (shot) for one of the divided exposure areas at a time. Thus, the exposing process is repeated the number of times corresponding to the number of the divided areas (namely, the process is performed in so-called step and repeat method). As a result, the exposure process is performed on the entire surface of the substrate. Consequently, the exposing device can expose a large area of a substrate that is greater than the area of the exposing device.
However, in an active matrix type liquid crystal display apparatus fabricated by the stepper method, when the same image signal is input to pixels in different exposure areas, the luminances thereof may be different from each other. In particular, when the difference of luminances in adjacent exposure areas is large, the boundary line of each exposure area is viewed as a "seam". Thus, the display quality of the active matrix type liquid crystal display apparatus that should display an image with high accuracy is remarkably deteriorated.
It is known that the difference of luminances in adjacent exposure areas results from the following reasons.
In other words, in a capacitive drive type display device such as a liquid crystal display device, a divided voltage of the input signal takes place between the capacitance of the pixel and a stray capacitance of the pixel. Thus, the voltage applied to the pixel is shifted by the divided voltage of the stray capacitance.
The amount of the stray capacitance depends on the overlap amount of each thin film pattern that constructs each pixel of the display device. When a photomask used for the exposing process of each thin film layer is aligned with an error against a predetermined position, a developed photoresist pattern and an etched thin film pattern deviate from their predetermined positions. Thus, the upper thin film pattern deviates from the lower thin film pattern. Consequently, the overlap amount of the fabricated device deviates from the designed overlap amount.
In addition, due to the accuracy of the drive portion of the exposing device, the deviation of a photomask in one exposure area (namely, mask alignment) may be different from that in another exposure area. Consequently, since the overlap amount and the stray capacitance in one exposure area are different from those in another exposure area, the voltage shift amount varies in each exposure area.
Thus, in the liquid crystal display device fabricated by the conventional fabrication method, there is a difference of luminances in different exposure areas, resulting in an irregular display image.
Experimental results of the display screen of the liquid crystal display device fabricated by the conventional fabrication method shows the following points. In particular, when the same image signal is input to pixels in two adjacent exposure areas, if the difference of transmittances of pixels in one area and pixels in another area is 0.5% or greater, the difference is viewed as a "seam" of the areas.
The present invention is made from the above-described point of view. An object of the present invention is to provide a liquid crystal display device having an equal display screen free of an obtrusive line-shaped luminance defect such as a "seam".